Nervous SystemsChapt 48 (pp 1011-1025)

4/21/06

IB-202-14-06

Complex Brain-Human!• Overview: Command and Control Center

• The human brain contains an estimated 100 billion nerve cells, or neurons

• Each neuron may communicate with thousands of other neurons

• Functional magnetic resonance imaging– Is a technology that can reconstruct a three-

dimensional map of brain activity

Figure 48.1

Colored areas of brain active during language processing.

The results of brain imaging and other research methods reveal that groups of neurons function in specialized circuits dedicated to different tasks

Simple Nervous Systems• Concept 48.1: Nervous systems consist of

circuits of neurons and supporting cells

• All animals except sponges have some type of nervous system

• What distinguishes the nervous systems of different animal groups is how the neurons are organized into circuits

• Most invertebrate nervous systems are simple

Organization of Nervous Systems• The simplest animals with nervous systems,

the cnidarians have neurons arranged in nerve nets

Figure 48.2a

Nerve net

(a) Hydra (cnidarian)

When prey touch a tentacle, the hydra can contract its tentacle to its mouth and engulf the prey item.

Star fish• Sea stars have a nerve net in each arm connected by radial

nerves to a central nerve ring. No Photosensitive Organs

Figure 48.2b

Nervering

Radialnerve

(b) Sea star (echinoderm)

Each radial nerve would have smaller nerves sending signals to the water vascular system as well as muscles.

Appearance of cephalization and centralization of nervous system

• In relatively simple cephalized animals, such as flatworms a central nervous system (CNS) is evident

Figure 48.2c

Eyespot

Brain

Nerve cord

Transversenerve

(c) Planarian (flatworm)

1st appearance of eye spots at head end! Allow it to turn away from light!

Two ventral nerve cords (interconnected so communicate with each other)!

Segmented invertebrates

• Annelids and arthropods– Have segmentally arranged clusters of neurons

called ganglia

• These ganglia connect to the CNS and make up a peripheral nervous system (PNS)

Brain

Ventral nervecord

Segmentalganglion

Brain

Ventralnerve cord

Segmentalganglia

Figure 48.2d, e (d) Leech (annelid) (e) Insect (arthropod)

Anteriornerve ring

Longitudinalnerve cords

Ganglia

Brain

Ganglia

Figure 48.2f, g (f) Chiton (mollusc) (g) Squid (mollusc)

Molluscs• Nervous systems in molluscs

– Correlate with the animals’ lifestyles• Sessile molluscs (clams sitting in the mud) have

simple systems while more complex molluscs have more sophisticated systems like the squid and octopus both which have eyes and are capable of complex behavior, including learning.

Well developed brain and eyes in squid!

Can “grasp” items with tentacles and manipulate them!

Vertebrates have a brain encased in a skull for protection.

• In vertebrates– The central nervous system consists of a brain and dorsal

spinal cord– The periferal nerves system connects to the CNS

Figure 48.2h

Brain

Spinalcord(dorsalnervecord)

Dorsal sensoryganglion

(h) Salamander (chordate)

What does the nervous system do? Gathers information about its surroundings, processes it and acts on it with

some sort of output.• Nervous systems process information in three

stages--sensory input, integration, and motor output

Figure 48.3

Sensor

Effector

Motor output

Integration

Sensory input

Peripheral nervoussystem (PNS)

Central nervoussystem (CNS)

Knee jerk as an example of info processing outside of the brain

• The three stages of information processing

Figure 48.4

Sensory neurons from the quadriceps also communicatewith interneurons in the spinal cord.

The interneurons inhibit motor neurons that supply the hamstring (flexor) muscle. This inhibition prevents the hamstring from contracting, which would resist the action of the quadriceps.

The sensory neurons communicate with motor neurons that supply the quadriceps. The motor neurons convey signals to the quadriceps, causing it to contract and jerking the lower leg forward.

4

5

6

The reflex is initiated by tapping

the tendon connected to the quadriceps

(extensor) muscle.

1

Sensors detecta sudden stretch in the quadriceps.

2 Sensory neuronsconvey the information to the spinal cord.

3

Quadricepsmuscle

Hamstringmuscle

Spinal cord(cross section)

Gray matter

White matter

Cell body of sensory neuronin dorsal root ganglion

Sensory neuron

Motor neuron

Interneuron

Stretching of quadriceps when leaning forward!

Information is in the form of electrical signals.

Neuron Structure• Most of a neuron’s organelles are located in the cell

body. Axons conduct impulse away from cell body!

Figure 48.5

Dendrites

Cell body

Nucleus

Axon hillock

AxonSignal direction

Synapse

Myelin sheath

Synapticterminals

Presynaptic cell Postsynaptic cell

• Most neurons have dendrites– Highly branched extensions that receive signals

from other neurons

• The axon is typically a much longer extension– That transmits signals to other cells at synapses– That may be covered with a myelin sheath

• Neurons have a wide variety of shapes– That reflect their input and output interactions

Figure 48.6a–c

Axon

Cell body

Dendrites

(a) Sensory neuron (b) Interneurons (c) Motor neuron

• Oligodendrocytes (in the CNS) and Schwann cells (in the PNS) are supporting cells that form the myelin sheaths around the axons of many vertebrate neurons

Myelin sheathNodes of Ranvier

Schwanncell Schwann

cellNucleus of Schwann cell

Axon

Layers of myelin

Node of Ranvier

0.1 µm

Axon

Figure 48.8

Basis for generation of anelectrical signal is the alteration of the resting

membrane potential of excitable cells!

• A membrane potential is a localized electrical gradient across membrane. The basis for the gradient is the disproportionate distribution of charged ions.– Anions are more concentrated within a cell.– Cations are more concentrated in the extracellular

fluid.– A greater number of negative charges within the cell

1. Every cell has a voltage, or membrane potential, across its plasma membrane

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

The resting membrane potential of a cell can be measured

Figure 48.9

APPLICATIONElectrophysiologists use intracellular recording to measure the membrane potential

of neurons and other cells.

TECHNIQUE A microelectrode is made from a glass capillary tube filled with an electrically conductive salt solution. One end of the tube tapers to an extremely fine tip (diameter < 1 µm). While looking through a microscope, the experimenter uses a micropositioner to insert the tip of the microelectrode into a cell. A voltage recorder (usually an oscilloscope or a computer-based system) measures the voltage between the microelectrode tip inside the cell and a reference electrode placed in the solution outside the cell.

Microelectrode

Referenceelectrode

Voltage recorder

–70 mV

• Concept 48.2: Ion pumps and ion channels maintain the resting potential of all cells including neurons

• For a neuron the resting potential is the membrane potential of a cell that is not transmitting signals

• Cells that can transmit signals are called excitable cells (nerves and muscles)

• How a Cell Maintains a Membrane Potential.– Cations.

• K+ the principal intracellular cation.

• Na+ is the principal extracellular cation.

– Anions.• Proteins, amino acids, sulfate, and phosphate are the

principal intracellular anions.

• Cl– is principal extracellular anion.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

• Ungated ion channels allow ions to diffuse across the plasma membrane.– These channels are always open.

• This diffusion does not achieve an equilibrium since sodium-potassium pump transports these ions against their concentration gradients. If poison the pump they will.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 48.7

Size of arrow represents the rate of diffusion. Faster for K+ than Na+

Excitable cells have the ability to generate large changes in their membrane potentials because they have gated ion channels.– Gated ion channels open or close in response to

stimuli. (These are separate and different from the ion channels in the former slide)• The subsequent movement of ions across the membrane

leads to a change in the membrane potential.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Gated Ion Channels

• Gated ion channels open or close– In response to membrane stretch or the

binding of a specific ligand– In response to a change in the membrane

potential

Production of Action Potentials

• In most neurons, depolarizations– Are graded only up to a certain membrane

voltage, called the threshold

• Some stimuli trigger a hyperpolarization– An increase in the magnitude of the membrane

potential

Figure 48.12a

+50

0

–50

–100

Time (msec)0 1 2 3 4 5

Threshold

Restingpotential Hyperpolarizations

Me

mb

ran

e p

ote

ntia

l (m

V)

Stimuli

(a) Graded hyperpolarizations produced by two stimuli that increase membrane permeability to K+. The larger stimulus producesa larger hyperpolarization.

• Other stimuli trigger a depolarization– A reduction in the magnitude of the membrane

potential

Figure 48.12b

+50

0

–50

–100

Time (msec)0 1 2 3 4 5

Threshold

Restingpotential

Depolarizations

Me

mb

ran

e p

ote

ntia

l (m

V)

Stimuli

(b) Graded depolarizations produced by two stimuli that increase membrane permeability to Na+.The larger stimulus produces alarger depolarization.

• Hyperpolarization and depolarization– Are both called graded potentials because the

magnitude of the change in membrane potential varies with the strength of the stimulus

• A stimulus strong enough to produce a depolarization that reaches the threshold of -55mV triggers a different type of response, called an action potential

Figure 48.12c

+50

0

–50

–100

Time (msec)0 1 2 3  4 5 6

Threshold

Restingpotential

Me

mb

ran

e p

ote

ntia

l (m

V)

Stronger depolarizing stimulus

Actionpotential

(c) Action potential triggered by a depolarization that reaches the threshold.

• An action potential– Is a brief all-or-none depolarization of a

neuron’s plasma membrane– Is the type of signal that carries information

along axons

• Both voltage-gated Na+ channels and voltage-gated K+ channels– Are involved in the production of an action

potential

• When a stimulus depolarizes the membrane– Na+ channels open, allowing Na+ to diffuse into the

cell changing the potential to a positive value

• As the action potential subsides– K+ channels open, and K+ flows out of the cell

• A refractory period follows the action potential– During which a second action potential cannot

be initiated

• An action potential can travel long distances– By regenerating itself along the axon

• The generation of an action potential

–  –  –  –  –  –  –  –

+  +  +  +  +  +  +  + +  + +  ++  +

–  – –  – –  –

+  +

–  –

+  +

–  –

+  +

–  –

+  +

–  –

+  +

–  –

+  +

–  –

+  +

–  –

+  +

–  –

+  +

–  –

+  +

–  –

+  +

–  –

+  +

–  –

–  –

+  +

–  –

+  +

–  –

+  +

–  –

+  +

Na+ Na+

K+

Na+ Na+

K+

Na+ Na+

K+

Na+

K+

K+

Na+ Na+

5

1 Resting state

2 Depolarization

3 Rising phase of the action potential

4 Falling phase of the action potential

Undershoot

1

2

3

4

5 1

Sodiumchannel

Actionpotential

Resting potential

Time

Plasma membrane

Extracellular fluid ActivationgatesPotassium

channel

Inactivationgate

Threshold

Mem

bran

e po

tent

ial

(mV

)

+50

0

–50

–100

Threshold

Cytosol

Figure 48.13

Depolarization opens the activation gates on most Na+ channels, while the K+ channels’ activation gates remain closed. Na+ influx makes the inside of the membrane positive with respectto the outside.

The inactivation gates on most Na+ channels close, blocking Na+ influx. The activation gates on mostK+ channels open, permitting K+ effluxwhich again makesthe inside of the cell negative.

A stimulus opens theactivation gates on some Na+ channels. Na+

influx through those channels depolarizes the membrane. If the depolarization reaches the threshold, it triggers an action potential.

The activation gates on the Na+ and K+ channelsare closed, and the membrane’s resting potential is maintained.

Both gates of the Na+ channelsare closed, but the activation gates on some K+ channels are still open. As these gates close onmost K+ channels, and the inactivation gates open on Na+ channels, the membrane returns toits resting state.

Conduction of Action Potentials

• An action potential can travel long distances– By regenerating itself along the axon

Figure 48.14

– +– + + + + +

– +– + + + + +

+ –+ – + + + +

+ –+ – + + + +

+ –+ – – – – –+ –+ – – – – –

– – – –– – – –

– –– –

+ +

+ +

+ ++ + – – – –

+ ++ + – – – –

– –– – + + + +– –– – + + + +Na+

Na+

Na+

Actionpotential

Actionpotential

ActionpotentialK+

K+

K+

Axon

An action potential is generated as Na+ flows inward across the membrane at one location.

1

2 The depolarization of the action potential spreads to the neighboring region of the membrane, re-initiating the action potential there. To the left of this region, the membrane is repolarizing as K+ flows outward.

3 The depolarization-repolarization process isrepeated in the next region of the membrane. In this way, local currents of ions across the plasma membrane cause the action potential to be propagated along the length of the axon.

K+

• At the site where the action potential is generated, usually the axon hillock– An electrical current depolarizes the

neighboring region of the axon membrane

Conduction Speed

• The speed of an action potential– Increases with the diameter of an axon

• In vertebrates, axons are myelinated– Also causing the speed of an action potential

to increase

Myelinated axons conduct impulses faster than non-myelinated

• Action potentials in myelinated axons jump between the nodes of Ranvier in a process called saltatory conduction

Cell body

Schwann cell

Myelin sheath

Axon

Depolarized region(node of Ranvier)

++ +

++ +

++ +

++

– –

– –

– –

–––

–

–

–

Figure 48.15

• Concept 48.4: Neurons communicate with other cells at synapses

• In an electrical synapse– Electrical current flows directly from one cell to

another via a gap junction (tail flick escape response in lobster uses electrical connection because it must be as fast as possible).

• The vast majority of synapses – Are chemical synapses

• In a chemical synapse, a presynaptic neuron releases chemical neurotransmitters, which are stored in the synaptic terminal

Figure 48.16

Postsynapticneuron body

Synapticterminalof presynapticneurons

5 µ

m

• When an action potential reaches a terminal– The final result is the release of

neurotransmitters into the synaptic cleft

Figure 48.17

Presynapticcell

Postsynaptic cell

Synaptic vesiclescontainingneurotransmitter

Presynapticmembrane

Postsynaptic membrane

Voltage-gatedCa2+ channel

Synaptic cleft

Ligand-gatedion channels

Na+

K+

Ligand-gatedion channel

Postsynaptic membrane

Neuro-transmitter

1 Ca2+

2

3

4

5

6

Action potential results in influx of calcium! Calcium causes vesicles to fuse with presynaptic membrane releasing neurotransmitter